In the animal kingdom, pheromones are scent signals that elicit specific behaviors or responses, including sexual arousal. But don't get too excited there, Brian Fantana, this doesn't mean it will work for humans. By the strict biological definition of pheromones, there is no good evidence that people have them — scent is a more subtle sexual indicator for us. "If we step outside of that very specific biological definition, then I think that scent and scent communication do play important roles in human sexuality," says Kelly Gildersleeve, a post-doctoral research fellow at Chapman University. Our bodily odors may not whip potential mates into a lustful frenzy, but some studies show that smells might color perceptions to a certain extent, possibly being a small part of what makes one person find another attractive. Here's what the science says.
The Scent of Compatibility
In experiments where women have been presented with men's natural body odors, there are certain traits that tend to rate as smelling better. The potential scent attraction that's received the most attention is that women seem to favor the smells of men who have immune genes that differ from their own. The theory is that women might be sniffing out men's major histocompatibility complex (MHC), a group of genes that effect the immune system. Following evolutionary theory, this makes sense. Women would want to mate with men who have different genes because that's more likely to result in offspring who can weather more diverse threats. Many studies have been done on MHC and scent preference, but Gildersleeve says evidence about whether it impacts partner choice is still mixed.
Other traits that may be communicated through smell include body and facial symmetry, testosterone levels, and behavioral dominance (assessed through a measure of narcissism). Within a meta-analysis of how women's partner preferences changed over their ovulatory cycle, Gildersleeve found that women at the high-fertility point of their cycle preferred scents associated with facial and body symmetry versus women at low fertility, but it was not a statistically significant difference.
Associations with testosterone are still shaky, too, but some research has indicated a relationship. A 2013 study published in Evolution and Human Behavior found that women in the fertile part of their cycle favor the scent of men with high levels of testosterone. There is one study that suggests that women in the fertile phase of their cycle preferred the scent of men who scored high on a dominance questionnaire, specifically as it relates to narcissism.
There is also plenty of research looking into whether men can smell the fertility of women and how that may create a response in hormone levels. A study from Frontiers in Endocrinology had 115 men smell the body odor and genital odor of 45 women, and found that the men's testosterone and cortisol levels increased in response to both odors if they came from fertile women, with the response lasting longer after smelling the genital odor. Testosterone and cortisol levels dropped if the men smelled the body odor of a woman who wasn't ovulating, and cortisol increased if it was the genital odor of a woman who wasn't ovulating. Overall, however, the effects of female smell on men is uncertain also.
How Attractiveness Becomes a Scent
We don't really know why traits might smell differently, but it's possible that it all relates to hormones. For example, our armpits have scent-producing organs that are dense with hormone receptors. These secrete all kinds of lovely chemicals, including alcohols, esters, and fats. It's possible, says Gildersleeve, that different mixes of hormones cause these organs to secrete different mixes of chemicals. The microflora (bacteria) on our skin and in our hair follicles eat those chemicals and give off odors. So, a change in chemicals could lead to a change in the odors the microflora give off.
If testosterone levels are detectable through scent, this could be why — and higher levels of testosterone have been associated with dominant behaviors, such as status seeking and possibly aggression, and might relate to facial and body symmetry too, but support for that is weaker.
The Effects of Birth Control
Much of the research done on the sex and scent looks at how women's reactions to smells differ over their ovulatory cycles. When women use certain kinds of birth control, such as the pill, their ovulation cycle and related hormones change. As a result, the scents they favor may also change. Research published in Hormones and Behavior assessed the abilities of 33 women to identify various smells, including so-called social odors, scents associated with chemicals that are found in larger quantities in men than in women and present in sweat and urine. This study found that women who were not on the pill and were in the fertile phase of their cycle were more sensitive to social odors than women on the pill. Although this is a small study, it adds to research suggesting that the pill does have some effect on women's sense of smell as it relates to body odor. It is important to note that hormonal contraceptives vary widely and each formulation may have its own effects.
This potential impact mirrors findings in some of the MHC studies. In certain research, such as this oft-cited 2008 study, women using hormonal contraception no longer preferred men who were MHC-dissimilar from themselves. So should a woman go off contraception to check that she is the right fit for her mate? "I think that that kind of sensationalized interpretation is not warranted," says Katy Renfro, doctoral candidate at Emory University in the Neuroscience and Animal Behavior Program and lead author of the Hormones and Behavior study. "If you love your partner and happened to meet your partner while on the pill, but then you go off the pill, you're not going to despise that person." There are many reasons why a woman might want to go off the pill or try a different kind of birth control, including a change in sexual desire, but concerns about the scent attractiveness of her partner is not a good one.
How to Smell Sexier
Do not buy into claims of perfumes that say they contain human pheromones. Science hasn't yet confirmed (or denied) the presence of pheromones in people, so it would be awfully difficult to bottle those up. At this point, we can't even specify what makes someone smell attractive to someone else. "I don't know exactly what a 'good' smell smells like," says Gildersleeve, "but the one thing I do know is that less intense odors tend to be rated as smelling better." However, don't go so far as to mask your natural scent completely. In her lab studies, Gildersleeve says people often react positively to a little body odor and she personally believes that people often have a certain affection for the way their partner smells naturally.